The present invention relates to the field of security printing, in general, as well as to the field of ink jet printing and compositions useful therein. This invention particularly relates to an aqueous latent image printing method and an aqueous latent image printing ink for use therewith.
In an aqueous latent image printing method, information is printed on a substrate, such as paper, which is transferred in the form of a latent image or xe2x80x9csecure imagexe2x80x9d which is invisible to the eye and any other usual image detecting device at the time of printing, and is revealed only after the substrate is subjected to a subsequent process of image activation.
Invisible inks have existed for many years. Many such ink systems have been found, developed and used in a limited way mainly because of the limited accessibility of such invisible inks and delivery systems for the latter.
A problem with the use of latent image printing for the purpose of product identification was that no good means existed for placing such information on a variety of substrates. Contact printing is not suitable for many such uses, as there is no good means to place the image on the desired substrate if it is irregularly shaped or is substantially inaccessible to traditional contact printing devices. Thus, latent image variable information such as production date, lot number, batch number, serial number, and the like, could not be placed on many products, except by hand, which is cumbersome, expensive, and prone to inaccuracies, defeating the purpose of using such information.
To address such problems, ink jet technology has been employed for security printing. Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure, through at least one orifice or nozzle. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. Since drop-on-demand systems require no ink recovery, charging, or deflection, the system can be much simpler than the continuous stream type.
There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway, having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on the open end and a heat generating resistor near the nozzle. The drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530.
U.S. Pat. No. 5,395,432 teaches the use of certain ink jet compositions for use in printing latent images an a substrate. In accordance with that patent, ink jet compositions comprising zinc chloride are taught for use in making latent images. A problem associated with those formulations, however, is their corrosiveness to metal parts of the printing apparatus with which the ink comes into contact.
U.S. Pat No. 5,935,308 purports to address this problem. The ""308 patent teaches the use of certain ink jet compositions for use in printing latent images on a substrate. In accordance with the ""308 patent, ink jet compositions comprising zinc bromide and preferably tetraethylammonium p-toluene sulfonate (xe2x80x9cTEA p-TSxe2x80x9d) are taught for use in making latent images.
It would be desirable to provide an ink jet composition for latent image printing that is not corrosive.
All references cited herein are incorporated herein by reference in their entireties.
The present invention provides an aqueous latent image printing method comprising applying to a substrate, such as paper, a latent image ink composition, via an ink jet printer. Subsequently, the latent image is activated to render it visible, by application of an activator.
Embodiments of the present invention are particularly suitable for security printing on a substrate which enables the free flow of information without the concern for fraud. The information that is printed on a substrate is invisible to the naked eye, and cannot be revealed without the use of an activator. This method is particularly useful for authentication and identification of financial transactions on the internet, such as checks, to secure the privacy of the user.